1. Field of the Invention
The present invention relates to a micro-strip transmission line capable of reducing far-end crosstalk, and more particularly, to a structure of a micro-strip transmission line having a serpentine shape for reducing far-end crosstalk of a transmission line by increasing capacitive coupling between neighboring transmission lines by allowing parallel micro-strip transmission lines to have serpentine shapes.
2. Description of the Related Art
Crosstalk is caused by electromagnetic interference between neighboring signal lines. When a high frequency signal is transmitted through two parallel long signal lines, signals transmitted through one or two signal lines mutually interfere with one another. A transmission loss is increased due to the crosstalk caused by the mutual interference.
Capacitive coupling caused by mutual capacitance and inductive coupling caused by mutual inductance occur between the two signal lines. Far-end crosstalk is caused by a difference between the capacitive coupling caused by the mutual capacitance and the inductive coupling caused by the mutual inductance.
FIG. 1 illustrates a structure of a conventional micro-strip transmission line.
In FIG. 1, two parallel micro-strip transmission lines 10 and 20 are shown. An end of each transmission line is terminated with a resistor having resistance R0 the same as the characteristic impedance of the transmission line.
A transmission line through which a signal is applied to an end (a sending end) thereof between two transmission lines is referred to as an aggressor line 10. The aggressor line 10 includes a voltage source vs1 and an applied voltage val. The other transmission line through which a signal is not applied is referred to as a victim line 20. Far-end crosstalk VFEXT of the victim line 20 may be represented by Equation 1 as follows:
                                                        V              FEXT                        ⁡                          (              t              )                                =                                    TD              2                        ·                          (                                                                    C                    m                                                        C                    T                                                  -                                                      L                    m                                                        L                    S                                                              )                        ·                                          ∂                                                      V                    a                                    ⁡                                      (                                          t                      -                      TD                                        )                                                                              ∂                t                                                    ,                            [                  Equation          ⁢                                          ⁢          1                ]            where, TD is a transmission time taken to transmit a signal through a transmission line, Cm is mutual capacitance per unit length, CT is a sum of self capacitance per unit length and the mutual capacitance, Lm is mutual inductance per unit length, and LS is self inductance per unit length. Here, Va(t) is a voltage applied to a sending-end of the aggressor line.
In case of a transmission line located in a homogeneous medium such as a strip line, a capacitive coupling amount is the same as an inductive coupling amount. Ideally, the far-end crosstalk becomes zero.
However, in case of a micro-strip line formed on a printed circuit board (PCB), the inductive coupling amount is greater than the capacitive coupling amount. Thus, the far-end crosstalk has a negative value. Although the strip-line transmission line can remove the far-end crosstalk, the strip-line transmission line has to use more layers of the PCB than the micro-strip line. Accordingly, costs are increased.
When independent signals are respectively applied to two parallel micro-strip lines, a case where two applied signals are changed to the same direction is referred to as an even mode, and a case where the two applied signals are changed to different directions is referred to an odd mode.
FIG. 2 illustrates concepts of even and odd modes. The combination of signals applied to a pair of coupled aggressor and victim transmission lines can be classified into three categories, as shown in FIG. 2. The even mode refers to the case where the two signals make transitions (i.e. 0, 1) in the same direction at a given instance of time. The odd mode refers to the case where the two signals make transitions in the opposite direction at a given instance of time (i.e. 0, 1 or 1, 0). The static mode refers to the case where one signal makes a transition (i.e. 0, 1) while the other signal does not change with time (i.e. 0 or 1).
Referring to FIG. 2, if an applied signal is increased with respect to time, far-end crosstalk has a negative pulse shape. Accordingly, the far-end crosstalk delays a signal change with respect to time in the even mode. On the contrary, the far-end crosstalk advances a signal change with respect to time in the odd mode.
That is, in the even mode, a signal transmission time is slightly increased. In the odd mode, the signal transmission time is slightly decreased. A difference in the signal transmission time between the even and odd modes is represented by Equation 2 as follows:
                                          TD            EVEN                    -                      TD            ODD                          =                  l          ·                                                    L                S                            ⁢                              C                T                                              ·                      (                                                            L                  m                                                  L                  S                                            -                                                C                  m                                                  C                  T                                                      )                                              [                  Equation          ⁢                                          ⁢          2                ]            where, 1 is a length of a transmission line, TDEVEN is a transmission time in an even mode, TDODD is a transmission time in an odd mode, Cm is mutual capacitance per unit length, CT is a sum of self capacitance per unit length and the mutual capacitance, Lm is mutual inductance per unit length, and Ls is self inductance per unit length.
FIG. 3 illustrates influences of crosstalk noise (shown by the dotted lines in FIG. 3) in even and odd modes.
Referring to FIG. 3, in a case where random data signals are applied to sending ends of two parallel micro-strip transmission lines, timing jitter due to a difference in rise-time at a receiving-end between even and odd modes which is caused by a difference in arriving time of signals between the even and odd modes occurs.
In methods of reducing the crosstalk effect occurring at the micro-strip transmission line, a spacing between signal lines is increased, or a guard trace is used. The guard trace is a structure for reducing coupling between neighboring signal lines by inserting a parallel trace between the signal lines. However, in the aforementioned methods, the micro-strip transmission lines occupy too large area in the PCB.